104 



OBELIA. PHYLUM CCELENTERATA 



polyp heads as hydranths. The whole colony is enclosed in a 

 perisarc of chitin (p. 159), which is secreted by the ectoderm and 



Fig. 69. — Part of a colony of Obelia, magnified. 



bL, Blastostyle ; ect., ectoderm ; end., endoderm ; gth., gonotheca ; hyth., hydrotheca; 

 med., medusa bud ; p.b., polyp bud ; perith., perisarc'. 



follows closely the outline of the body, but is separated from it 

 by a small space, bridged by processes from some of the ectoderm 

 cells. At the base of each hydranth the perisarc expands into a 

 cup or hydrotheca into which the hydranth can be withdrawn. 



